Lithium ion secondary cells are widely used as small cells for portable electronic devices or personal computers, high-capacity large chargeable/dischargeable cells for power sources such as electric vehicles (EVs), hybrid vehicles (HEVs) and plug-in hybrid vehicles (PHVs), and power storage systems for large-scale energy storage and power supplies for large-scale disasters, and ultra-large charge/discharge cells used in electric power substation forming electrical grids called “smart grids”. Lifespan required for lithium ion secondary cells for small electronic devices such as portable electronic devices is 1 to 3 years, while lengthened lifespan of 10 to 20 years is required for large lithium ion secondary cells. Furthermore, lifespan of at least 25 to 30 years is required for ultra-large lithium ion secondary cells. For this reason, little deterioration in cell capacity upon repeated charge/discharge and high maintenance of cell capacity, that is, superior charge/discharge cycle characteristics, are required. A lithium ion secondary cell has a basic structure in which a positive electrode active material layer containing a positive electrode active material and a negative electrode active material layer containing a negative electrode active material, which are respectively formed on current collectors, face each other via a separator, and the positive and negative electrode active material layers are immersed in an electrolytic solution and these components are accommodated in an outer package. Regarding the lithium ion secondary cell having such a structure, each of the positive electrode active material and the negative electrode active material reversibly intercalates and deintercalates lithium ions, thereby performing charge/discharge cycles.
As lithium ion secondary cells with long lifespan, lithium ion secondary cells which improve cycle characteristics by using lithium manganese-based composite oxide and lithium nickel-based composite oxide having a three-dimensional host structure as positive electrode active materials are reported in Patent Document 1.
Also, lithium ion secondary cells wherein cyclic sulfonic acid ester containing at least two sulfonyl groups is contained in an electrolytic solution, a surface film that is solid electrolyte interphase; SEI film for suppressing deterioration caused by charge/discharge is formed on a negative electrode surface, release of manganese from manganese oxide contained in a positive electrode is suppressed and adhesion of manganese to the negative electrode surface is suppressed and charge/discharge cycle characteristics are improved in Patent Document 2, lithium ion secondary cells wherein a thin passivation film is formed on the interface between the positive electrode and the electrolyte by preliminarily thermally treating a positive electrode using lithium manganese spinel in an electrolytic solution in a discharge state and release of Mn is thus suppressed, thereby improving coulomb efficiency and cycle characteristics and storage characteristics at high temperatures in Patent Document 3, and secondary cells wherein the cyclic sulfonic acid ester contained in the electrolytic solution is decomposed by charging and then aging, the sulfur-containing protective film is formed on the positive electrode, and rapid charge/discharge cycle lifespan at high temperatures are improved in Patent Document 4 or the like were reported. Alternatively, lithium ion secondary cells which improve current collection and rate characteristics or cycle characteristics by incorporating carbon nanotubes as the conductive material to form a low-resistance conductive network in the positive electrode containing manganese-based composite oxide as an active material in Patent Document 5 was reported.